1. Field of the Invention
The present invention relates to a hydraulic braking apparatus for a vehicle wheel wherein the hydraulic pressure in a brake cylinder for the wheel is electrically controlled, and more particularly to improvements in operating reliability of such hydraulic braking apparatus.
2. Discussion of the Prior Art
For braking a wheel of a motor vehicle, there is known a hydraulic braking apparatus in which the hydraulic pressure generated by a hydraulic pressure source is electrically controlled to a level corresponding to an operating amount of a brake operating member such as a brake pedal, and the thus controlled hydraulic pressure is applied to a brake cylinder for braking the vehicle wheel. An example of such hydraulic braking apparatus is disclosed in JP-A-63-20256 (published in 1988). This braking apparatus has a spool-type solenoid-operated pressure control valve.
Such spool-type solenoid-operated pressure control valve includes a spool which is slidably and substantially fluid-tightly received in a bore formed in a valve housing, and a force motor or solenoid whose coil is energized to produce an electrically controllable pilot force, so that the pilot force acts on the spool in one direction. The pressure control valve is adapted such that a reaction force or feedback force proportional to the output hydraulic pressure of the control valve acts on the spool in the direction opposite to the direction in which the pilot force acts on the spool. This pressure control valve is capable of regulating the hydraulic pressure of an appropriate hydraulic pressure source to a level proportional to the amount of an electric current applied to the coil of the force motor or solenoid.
The valve housing of the spool-type solenoid-operated pressure control valve indicated above has a high-pressure port connected to the hydraulic pressure source, a low-pressure port connected to a reservoir, and an output port connected to a brake cylinder for braking a vehicle wheel. The pilot force acts on the spool in the direction for fluid communication of the output port with the high-pressure port. The feedback force is generated based on the hydraulic pressure at the output port, and acts on the spool in the direction for fluid communication of the output port with the low-pressure port. The spool is moved to a position of equilibrium between the pilot force and the feedback force, whereby the hydraulic pressure of the hydraulic pressure source is controlled according to the amount of electric current applied to the coil. On the other hand, the force acting on the brake pedal (hereinafter referred to as "pedal depression force" ) is detected by a suitable force sensor, while the deceleration value of the vehicle is detected by a deceleration sensor. The electric current for energizing the coil is determined so that the hydraulic pressure to be applied to the brake cylinder permits the detected vehicle deceleration value to coincide with a desired value which corresponds to the pedal depression force. This manner of controlling the pressure of the wheel brake cylinder is called herein "braking-effect monitoring brake control" . In a modified arrangement, the hydraulic pressure in the wheel brake cylinder is detected, so that the energization current applied to the coil is controlled to regulate the detected hydraulic pressure to a desired level that establishes the desired vehicle deceleration value corresponding to the pedal depression force.
In the known hydraulic braking apparatus using the spool-type solenoid-operated pressure control valve constructed as described above, the hydraulic pressure of the wheel brake cylinder is electrically controlled on the basis of an output signal of the sensor such as the vehicle deceleration sensor or hydraulic pressure sensor. Upon failure of the sensor, however, the hydraulic pressure of the wheel brake cylinder would not be adequately controlled, that is, the controlled hydraulic pressure would be excessively high or low in relation to the operating force acting on the brake pedal or brake operating member.